Method of and solution for accelerating activation of plastic substrates in electroless metal plating system

ABSTRACT

A method is disclosed for accelerating the activation of a polymeric plastic substrate in a chemical plating process incorporating a preliminary chromic acid etch involving immersing the substrate in an acidic dilute palladium chloride solution following activation, and rejuvenating that accelerating solution with periodic additions of stannous ions sufficient to reduce hexavalent chromium contamination carried over from the preliminary chromic acid etch to a prescribed maximum level without incurring substantial precipitation of palladium in the accelerating solution.

United States Patent Inventors Eugene D. DOttavio Waterbury, Conn.

METHOD OF AND SOLUTION FOR ACCELERATING ACTIVATION OF PLASTIC SUBSTRATESIN ELECTROLESS METAL PLATING SYSTEM 12 Claims, No Drawings US. Cl 117/47A, 106/1, 117/160 R, 204/30 Int. Cl B44 1/0 92, C23c 3/02 Field ofSearch 1 17/47 R,

Primary ExaminerAlfred L. Leavitt Assistant Examiner-Janyce A. BellAttorneys-Steward and Steward. Merrill F. Steward and Donald T. StewardABSTRACT: A method is disclosed for accelerating the activation of apolymeric plastic substrate in a chemical plating process incorporatinga preliminary chromic acid etch involving immersing the substrate in anacidic dilute palladium chloride solution following activation, andrejuvenating that accelerating solution with periodic additions ofstannous ions sufficient to reduce hexavalent chromium contaminationcarried over from the preliminary chromic acid etch to a prescribedmaximum level without incurring substantial precipitation of palladiumin the accelerating solution.

METHOD OF AND SOLUTION FOR ACCELERATING ACTIVATION OF PLASTIC SUBSTRATESIN ELECTROLESS METAL PLATING SYSTEM This invention is concerned withmetal plating of polymeric resin substrates, and is more particularlydirected to improvements in that portion of the metallizing processinvolving the laying down of an initial deposit or film of a desiredmetal, which deposit is continuous in its coverage of the substrate andis firmly bonded thereto.

The art of metallizing plastics both for decorative and functionalapplications is undergoing rapid development. The general methodemployed consists of first chemically depositing on a polymeric plasticsubstrate a preliminary conductive metallic coating after which thesubstrate can then be electroplated by standard electrochemicaltechniques. The successful application of the initial conductivemetallic layer is of crucial importance to the subsequent successfulelectroplating of one or more layers of the same or different metal inbuilding up a total metallic deposit of desired thickness and finishcharacteristics. Copper, nickel, chromium and sometimes cobalt are themetals most commonly applied to commercial articles, and the process isused extensively in the automotive, appliance, plumbing and relatedindustries today.

There are various methods in regular use for metallizing plastics. Mostcomprise a first or chemical, i.e. electroless, plating cycle of stepswhich consist of immersing the object to be plated in a number ofaqueous or nonaqueous solutions, with each of such solutionscontributing to a specific alteration of the polymer surface of thesubstrate to cause it to accept a thin and adherent initial metal layer.Thereafter a second processing phase or sequence of steps is involvedduring which thicker metal deposits are applied electrochemically, thatis, by the application of an external source of electrical energy. Inthe course of these operations, the substrates or articles to be platedare sequentially subjected to the various treatment solutions, as byspraying them with or immersing them in such solutions. In a commercialinstallation where mass production techniques are employed, the articlesare generally supported from racks carried by endless conveyor means, bywhich the articles are progressed through the sequence of treatmentsteps, including the chemical or electroless plating cycle as well asthe final electroplating operations.

This invention is concerned primarily with improvements in the chemicalor electroless plating cycle of operations, and it will be appropriatetherefore, as background for the invention here disclosed, to considerfirst of all the sequence of operating steps involved in a typical,current procedure. In general, the steps in chemical plating of apolymeric plastic substrate comprise the following:

. Clean the plastic part of surface grime, etc., in an aqueous alkalisoak solution.

2. Contact the cleaned part with an organic solvent medium which may beeither a single-phase system or an admixed water-organic solventemulsion. An example of such material and treatment is disclosed in acopending application, Ser. No. 654,901, filed June 14, 1967.

3. Thorough water rinse of the part.

4. Contact of the part with an aqueous acid solution containinghexavalent chromium ions to etch the surface of the plastic. An exampleof such treatment is disclosed in U.S. Pat. No. 3,370,974; a differentexample is disclosed in a copending application, Ser. No. 474,198, filedJune 25, 1965.

5. One or more rinses in water and/or solutions containingchromium-reducing or chromium-extracting agents. An example of this isdisclosed in a copending application Ser. No. 758,589, filed Sept. 9,1968.

6. Contact the surface of the substrate with an acid tin-palladiumhydrosol using a one-step activator. An example is found in U.S. Pat.No. 3,011,920; also a further modification is disclosed in U.S. Pat. No.3,532,518.

7. Again carefully rinse the surface.

8. Accelerate the activated surface of the plastic, using a dilutesolution of an acid or alkali. Examples of suitable solutions are shownin U.S. Pat. Nos. 3,01 1,920 and 3,532,518.

9. Rinse in water.

10. Immerse or otherwise contact the substrate surface with a chemicalplating solution containing a reducible salt of the metal to bedeposited on the surface. Examples are found in the patent andapplication listed in Step 6 above; also in U.S. Pat. Nos. 3,212,918 and3,370,974 for nickel and cobalt; also 3,095,309 for copper.

l 1. Rinse the metallized surface in water, which is now ready forconventional electroplating.

While the foregoing general sequence of steps is employed commercially,there are certain deficiencies in the process which have manifestedthemselves. Small changes in operating procedures or ambient conditionstend to give rise to difficulties, the cause or reason for which isoften unknown or hard to recognize since the art of plating on plasticsis still largely empirical. This is particularly true in commercialoperation where conditions are far less accurately controllable than inlaboratory tests.

One of the most important problems is plating skip" or lack ofcontinuity in the coverage of the substrate by the deposited metal. Thisis commonly noted on areas of the plastic which are highly stressed dueto the method of molding the particular part. Plating skips often occurin other nonstressed areas also. Poor adhesion is another importantproblem, and often this does not show up until a late stage in theprocess, usually not until after completion of the metallizingprocedure. All of this adds to the expense because the part must then bescrapped. Quite frequently these deficiencies arise from completelyunrecognized conditions which further complicate the process.

It is know, of course, that any surface contamination of the substrateat the time it is introduced into the chemical plating solution plays animportant role in the success of the plating operation and gives rise tothe type of difficulties mentioned above. It is also known thathexavalent chromium, which is present in the usual etching steppreceding activation, is a potent deterrent or poison to good depositionof the desired metal coating on the substrate. It is common practice,accordingly, to incorporate after the etching step and prior to thechemical plating operation one or more thorough water rinses or othertreatment solutions containing chrome-reducing agents to kill residualhexavalent chromium ions that may be present on the substrate. Obviouslysuch preactivation chrome-kill treatments must not also act to impedethe activation of the substrate surface. There is always too a problemof simple mechanical trapping of trace amounts of hexavalent chromium increvices or other inaccessible areas where the substrate article has acomplex shape or configuration.

It has now been discovered and it is a central concept of the presentinvention that many of the difficulties previously encountered withregard to skipping and poor adhesion of chemically deposited metalplates on polymeric substrates can be reduced or eliminated by the useof a special accelerating solution composition and procedure followingactivation of the plastic surface. Before discussing the presentinvention in detail, a brief discussion of accelerating procedures willbe helpful to a better understanding of the invention.

Accelerating a substrate surface after its activation is of course agenerally well'known procedure. The use of this step is postulated onthe assumption that during activation of the substrate not only ispalladium (or other catalytic metal) laid down to provide the desiredinitiating foci for the reduction of metal ions in the plating solution,but excess stannous ions and/or other impurities which are also presentin all of the commercial activating solutions, are also deposited on orat least adhere to the surface of the substrate. These stannous ions andother impurities are deterrents to subsequent deposition of metal, justas are the residual adherent hexavalent chromium ions mentioned above,and must be removed. This removal of impurities is the primary functionof the usual accelerating solution. Ordinary water rinses have somebeneficial effect but commonly the practice has been to incorporate anacid, such as hydrochloric or sulfuric, or to use perchloro or perfluorocompounds, to speed up the process. The problem is one of promotingremoval of these poisons preferentially to palladium particles or foci,since the latter are necessary as catalyzing sites. The knownaccelerating solutions are quite effective in removing stannous ions buttheir use is critical in that they must be accurately controlled toavoid also removing excess amounts of the palladium and thus impedingsuccessful or adequate plating on that account. The presence at thisstage of hexavalent chromium ions at extremely low levels has not beenrecognized heretofore as a source of trouble, and the usual acceleratingtreatments are largely ineffective in respect to this problem.

To overcome these deficiencies, it has now been found that effective andmore readily controllable acceleration can be accomplished with the useof an acidic dilute palladium chloride solution following activation. Ithas also been found, however, that the effectiveness of thisaccelerating solution is seriously impeded by the very low-level ortrace amounts of hexavalent chromium ion carried in by the substratefrom the prior etching operation. The level of hexavalent chromic ioncontamination found to be critical in this accelerator is unusually low,being on the order of one part per million, otherwise it will interfereand cause plate skipping and/or poor adhesion. It is therefore animportant part of this invention to control the hexavalent chromium ioncontamination of the accelerating solution by the periodic addition of asoluble source of stannous ions and thereby to effect reduction of thehexavalent chromium to trivalent condition before such critical point isreached. Since the addition ofstannous ions to the palladium chlorideaccelerating solution gives rise to a competing reaction, namely theundesirable reduction of palladinous ions to colloidal or particulatemetal, the addition is preferably effected in dilute solution and insuch total amount as to avoid substantial formation of particulatepalladium in the accelerating solution, certainly always insufficient toeffect stochiometric reduction of the palladium chloride. The use of adilute stannous solution avoids high local ion concentration at thepoint of addition, which could cause palladium deposi tion even thoughthe total amount of stannous ions present in the accelerating solutionwould not produce significant amounts of particulate palladium. Withinthese limits there is a preferential reaction of the stannous ions withhexavalent chromium, which affords reasonable leeway in effecting theselective reduction of that contaminant.

The invention is illustrated by the following example.

EXAMPLE I A molded polypropylene (Shell 5520 substrate, previouslycleaned of surface grime and grease in a mild aqueous alkali proprietarycleaner solution was plated with a continuous, firmly adherent nickeldeposit in the following manner:

a. immerse substrate in aqueous emulsion of an organic preconditioningagent comprising approximately, per liter of solution, 40 mls. ofsteam-distilled turpentine emulsified in water with surfactants. Thisemulsion and its preparation is more fully disclosed in copendingapplication Ser. No. 654,901, filed June 14, 1967. Immersion of the partin this solution is maintained for approximately 5 minutes at atemperature of 1501 60 F.

b. Cold water rinse.

c. Etch five minutes in a chromic-sulfuric at a temperature of l75l 80F.

d. Double-cold water rinses in separate rinsing tanks.

e. Immerse substrate in an aqueous solution containing controlledamounts of surfactants. Compositions useful in this step are shown moreparticularly in copending application Ser. No. 758,589, filed Sept. 9,1968.

f. Cold water rinse.

acid aqueous bath g. Immerse substrate in an acid tin-palladium hydrosolactivator solution containing the palladium in particulate or colloidalcondition. Immersion is maintained for about 3 minutes in such solutionat room temperature.

h. Cold water rinse.

i. Immerse the activated substrate for l to 3 minutes in an aqueousaccelerating solution at about 120 F. containing approximately 0.087,g./l palladium chloride and sufiicient hydrochloric acid to lower the pHof the solution to about 0.5

j. Cold water rinse.

k. Immerse substrate in any of the standard commercial electrolessnickel plating baths for 5 minutes at a bath temperature of -90 F.

l. Cold water rinse.

m. Electroplate using conventional procedures.

The resulting nickel deposit is smooth, bright and completely continuousin coverage of the substrate, including such difficulty platable areasas locations corresponding to gating points in the mold, or where thesurface configuration of the substrate produces deep crevices orrelatively inaccessible pockets.

Copper plating of the substrate in place of nickel can similarly beeffected with equally good result simply by substituting a commercialelectroless copper plating solution for the nickel in Step (k) of theforegoing cycle, all other steps being unchanged. A typical copperplating solution is disclosed in U.S. Pat. No. 3,095,309. The system isalso effective for electroless plating of cobalt, using any of thecommercially available electroless cobalt plating solutions.

The adverse effect upon complete metal coverage of the substrate articledue to the presence of minute or trace amounts of hexavalent chromiumion carried into the palladium chloride accelerating solution by thearticles themselves is demonstrated by the following.

EXAMPLE II An accelerating solution identical with that used in Step (i)of example I is purposely contaminated with incremental additions ofhexavalent chromium ion in controlled amounts, and identicalpolypropylene samples are plated in accordance with the previouslydescribed procedure at each of the different hexavalent chromium ionconcentration levels in the accelerating solution. It is found that theaddition of up to a total of 0.5 parts per million p.p.m. to theaccelerating solution produces no noticeable effect upon the resultingnickel or copper plate, the deposit still being complete and showing noevidence of skipping. However, upon further addition, at approximately alevel of 0.6 p.p.m. (total) hexavalent chromium ion in the acceleratingsolution, skipping or misplating of the part results.

EXAMPLE III A test similar to that described in example II was run butin this instance the concentration of the palladium chlorideaccelerating solution was increased to a level of approximately 0.0261percent (wt.) palladium chloride. The incremental addition to thisaccelerating solution of hexavalent chromium ion in 0.5 p.p.m. amountsproduced no evidence of skipping after a total of 1.0 p.p.m. ofcontaminant had been added; but severe skipping did occur when the levelreached l.5 p.p.m. hexavalent chromium ion.

Further increase in concentration of the palladium chloride appears toresult in proportional increase in tolerance of the acceleratingsolution to hexavalent chromium ion, but for practical purposes itbecomes uneconomical to operate the accelerating solution at palladiumconcentrations higher than 0.03 percent (wt.). In the other direction,effective results generally require a minimum palladium chlorideconcentration of the order of 0.002 percent. Although lowerconcentrations will work, solution temperature must be increased in suchcase and the tolerance to hexavalent chromium becomes critically low.

Before the contaminant level of the accelerating solution reaches themaximum tolerable state, the solution can be readily rejuvenated by theaddition of a soluble source of stannous ions to reduce the hexavalentchromium to trivalent condition. This latter ion does not interfere orcause plating skip. This is illustrated by the following.

EXAMPLE lV Again the same cycle of plating operation is followed asgiven in example I, and the palladium chloride accelerating solutiondeliberately contaminated in this case with 2 p.p.m. of hexavalentchromium ions. At this level of contaminant, almost no plated deposit isobtained on the substrate. With incremental additions of stannous ions,as dilute stannous chloride in hydrochloric acid solution, severeskipping in the plated deposit of the substrate continues to appearuntil a total of 35 p.p.m. of stannous ion has been added to theaccelerating solution. At this point, all skipping disappears andcomplete coverage of the surface of the substrate by the plating metalis obtained. The reaction involved in the addition of the stannous ionappears to be reduction of the hexavalent chromium ion to trivalentcondition, with simultaneous oxidation of the stannous to the stanniccondition. The chromium ion poisons the plating operation only whenpresent in the hexavalent form, as it has been found that the presenceof trivalent chromium up to at least 650 p.p.m. in the acceleratingsolution produces no adverse effects on the coverage or adhesion of theplate. However, the manner of adding the stannous ions to theaccelerating solution is important in order to avoid simultaneousreduction of palladinous ions to particulate palladium and deteriorationof the accelerating solution for that reason. For example, the additionof 0.1 ml. of a rejuvenating solution containing 0.84 pounds of stannouschloride, 3.96 pounds of concentrated hydrochloric, balance water, pergallon of total solution, results in immediate fonnation of substantialparticulate palladium in the accelerating solution. Still, 0.1 ml. ofthat solution is effective in countering the adverse effect of 2 p.p.m.of hexavalent chromium ion in 1 liter of accelerating solution, in thatcomplete plating coverage of the substrate article is obtained. A 1 ml.portion of a substantially more dilute stannous chloride solution (5percent by volume of the rejuvenating solution just mentioned, balancewater) when added to one liter of palladium chloride acceleratingsolution containing 2 p.p.m. of hexavalent chromium ion is alsoefiective to suppress the contaminating effect and to get completeplating coverage of substrate articles. Even at this diluted level ofstannous ion, formation of some particulate palladium will occur.Further dilution of the stannous rejuvenating solution to 3 percent(volume) is effective to suppress the contaminating effect of 2 p.p.m.of hexavalent chromium ion in the accelerating solution when about 3 ml.of this diluted solution is added to the accelerating solution; but evenhere slight amounts of particulate palladium form. This undesirable sidereaction can, however, be completely avoided by utilizing a 1 percent(volume) solution of the rejuvenating stannous chloride first mentioned.The hydrochloric acid level in such solution is about 0.05N. In apalladium chloride accelerating solution containing 2 p.p.m. hexavalentchromium ion as contaminant, the plating skip occasioned by thecontaminate will disappear upon the addition of 7 ml. per liter of the 1percent rejuvenating solution, and no particulate palladium formation isobserved in this case. The operative range of solutions comprises fromabout 0.05 to 0.3 percent by weight (0.5 to 3.0 g./l) stannous chlorideand 0.05N to 0.15N hydrochloric acid.

The rejuvenating effect of the stannous ion upon the acceleratingsolution appears to be unique in that other common reducing agents donot overcome the poisoning effect of hexavalent chromium ion. Forexample, in an accelerating solution containing 2 p.p.m. of hexavalentchromium as contaminant, the addition of as much as 24 ml. per liter ofhydrogen peroxide (30 percent) sill is not effective to overcome thecomplete inhibition by the contaminant of any metal deposit upon thesubstrate.

Hexavalent chromium ion is not the only trace contaminant or impurity inthe accelerating solution which will inhibit the satisfactory deposit ofa metal plate on the substrate, although it is the principal one usuallyencountered by reason of the preceding chromic-sulfuric acid etch stepin the plating cycle. For example, ferric ions in the acceleratingsolution will also produce serious skipping in the plated deposit. Inthis case it is generally found that the level at which skipping beginsto occur is on the order of 5-6 p.p.m. of ferric ion. The inhibitingaction of this contaminant can likewise be countered by the addition ofstannous ions to the accelerating solution to maintain the ferric ionconcentration below the aforesaid level of 56 p.p.m.

Thus it appears that, in relation to the palladinous ion concentrationin the accelerating solution, maintaining the hexavalent chromium ionconcentration at a. ratio not greater than about I p.p.m. to 145 p.p.m.ofpalladinous ion, by the addition of the stannous rejuvenatingsolution, is effective in maintaining the operability of theaccelerating solution.

The improvement afforded by the present invention is applicable tosubstrates other than polypropylene mentioned in the foregoing examples.The plating of phenolic, epoxy and polysufone polymers, as well ascopolymers, such as acrylonitrile-butadiene-styrene is similarlyimproved, and it appears that the invention is applicable to any of theusual chemically platable plastics.

We claim:

1. in a chemical plating process for depositing a continuous, adherentmeta] film on the surface of a polymeric plastic substrate, includingthe steps of etching the substrate surface in a chromic acid containingbath, activating the substrate surface by immersion in an acidtin-palladium hydrosol, subjecting the activated surface to contact withan accelerating solution to reduce excess stannous ions codeposited withpalladium during the activating step and thereafter immersing thesurface in a chemical plating solution of the metal to be deposited, theimprovement which comprises: employing in said accelerating step anacidic dilute palladium chloride solution, periodically adding to saidaccelerating solution an aqueous rejuvenating solution consistingessentially of a dilute source of stannous ions in soluble formsufficient to reduce the hexavalent chromium ion contamination carriedinto said accelerating solution from the etching step to a level not inexcess of about 1 p.p.m. hexavalent chromium per 145 p.p.m. palladinousion 'present in said solution.

2. A chemical plating process as defined in claim 1, wherein thepolymeric plastic substrate is selected from the group consisting ofacrylonitrile-butadiene-styrene, polypropylene, phenolic, epoxy andpolysulfone plastics.

3. A chemical plating process as defined in claim 1, wherein saidaccelerating solution contains about 0.002 to 0.03 percent by weight ofpalladium chloride and hydrochloric acid to give a pH of not over about1.0; said substrate being maintained in said accelerating solution atabout F. for l to 3 minutes.

4. A chemical plating process as defined in claim 1, wherein saidrejuvenating solution consists essentially of about 0.05 to 0.03 percentby weight stannous chloride in 0.05 to 0.15N. hydrochloric acid.

5. A chemical plating process as defined in claim 4, wherein therejuvenating solution contains from 0.1 to 0.2 percent by weight ofstannous chloride.

6. An aqueous accelerating solution for a chemical plating process whichconsists essentially of from about 0.002 to 0.03 percent by weightpalladium chloride, hydrochloric acid to provide a solution pH not overabout 1.0, and from about 0.05 to 0.3 percent by weight stannouschloride but always insufficient in amount to effect stochiometricreduction of the palladium chloride, wherein any hexavalent chromium ionis not in excess ofabout p.p.m.

7. In a chemical plating process for depositing a continuous, adherentmetal film on the surface of a polymeric plastic sub strate, includingthe steps of etching the substrate surface in a chemical etchantsolution, activating the substrate surface by 10. The process of claim 7wherein the said accelerating solution contains about 0.002 to 0.03 percpalladium chloride and h over about 1.0.

11-. The process of claim 7 wherein is polypropylene.

ent by weight of ydrochloric acid to give a pH of not the said plasticsubstrate 12. The process of claim 7 wherein the accelerating solutionis maintained at a temperature of about F. and the time plasticsubstrate therein is from 1 to 3 of immersion of the minutes.

2. A chemical plating process as defined in claim 1, wherein thepolymeric plastic substrate is selected from the group consisting ofacrylonitrile-butadiene-styrene, polypropylene, phenolic, epoxy andpolysulfone plastics.
 3. A chemical plating process as defined in claim1, wherein said accelerating solution contains about 0.002 to 0.03percent by weight of palladium chloride and hydrochloric acid to give apH of not over about 1.0; said substrate being maintained in saidaccelerating solution at about 120* F. for 1 to 3 minutes.
 4. A chemicalplating process as defined in claim 1, wherein said rejuvenatingsolution consists essentially of about 0.05 to 0.03 percent by weightstannous chloride in 0.05 to 0.15N. hydrochloric acid.
 5. A chemicalplating process as defined in claim 4, wherein the rejuvenating solutioncontains from 0.1 to 0.2 percent by weight of stannous chloride.
 6. Anaqueous accelerating solution for a chemical plating process whichconsists essentially of from about 0.002 to 0.03 percent by weightpalladium chloride, hydrochloric acid to provide a solution pH not overabout 1.0, and from about 0.05 to 0.3 percent by weight stannouschloride but always insufficient in amount to effect stochiometricreduction of the palladium chloride, wherein any hexavalent chromium ionis not in excess of about p.p.m.
 7. In a chemical plating process fordepositing a continuous, adherent metal film on the surface of apolymeric plastic substrate, including the steps of etching thesubstrate surface in a chemical etchant solution, activating thesubstrate surface by immersion in an acid tin-palladium hydrosol,accelerating the activated surface and thereafter immersing the surfacein a chemical plating solution of the metal to be deposited, theimprovement which comprises conducting the accelerating step byimmersing the plastic substrate in an acidic dilute palladium chloridesolution.
 8. The process of claim 7 wherein the polymeric plasticsubstrate is selected from the group consisting ofacrylonitrile-butadiene-styrene, polypropylene, phenolic, epoxy andpolysulfone plastics.
 9. The process of claim 7 wherein the plasticsubstrate surface is etched in a chromic acid containing bath.
 10. Theprocess of claim 7 wherein the said accelerating solution contains about0.002 to 0.03 percent by weight of palladium chloride and hydrochloricacid to give a pH of not over about 1.0.
 11. The process of claim 7wherein the said plastic substrate is polypropylene.
 12. The process ofclaim 7 wherein the accelerating solution is maintained at a temperatureof about 120* F and the time of immersion of the plastic substratetherein is from 1 to 3 minutes.